As David James the goalkeeper strides towards me, about to tell me all about his conversion to ecowarrior, I can't help thinking that the green movement has lucked out. Once derided for its sandal-wearing worthiness, it has somehow enlisted this 6ft 5in England-capped ex-Armani model: a serious coup for the planet. There is little denying that David James, 36, who this season broke the clean-sheet record for the Premiership, has a very long reach - both physically, for which he is highly feted and extremely well paid - and metaphorically. If you believe climate change is all about persuading enough people to change their behaviour, then this type of reach becomes highly significant.

But while the long arm of football fame is definitely useful, it shouldn't invite complacency. And at the same time that I see David emerging from the large Devon villa that he is currently restoring, I spot two large bags of Irish moss peat propped up against the wall. 'This is not good David,' I say in a rather more schoolmistressy tone than I intended. 'Let's talk to Paul,' he says, unfazed, pacing down the garden. 'Paul,' he shouts to his gardener, 'you can't use peat any more. Tell him why.' He ushers me forward to explain to Paul, and listens intently as I babble on about depleting Irish bogs, biodiversity and carbon sinks. Paul looks like he wishes I'd go home.

The peat crisis averted, we do a tour of the house and grounds. The last owner pretty much allowed the house to fall down, decamping from room to room as the rain came in through the roof. At the moment James is researching carbon-neutral heating systems, particularly woodchip fuel boilers, while he, his fiancee Amanda and her two children camp out in a flat at the back of the house. He's having the swimming pool filled in - 'too energy-intensive to run, and too dangerous,' he says - and the waste wood from the rebuild is to be recycled. 'Do you know how much waste is generated from construction?' he asks me. 'It's immense.'

I don't know many footballers, but I suspect that for them these topics of conversation are unusual. But then David James is known for being a bit unusual. There are the strange hair-dos (the Clark Kent comb-over), there's the art (James is a keen painter and his Jamaican father was an artist), and he admits to an obsession for collecting ephemera (Chopper bikes). The fact that he tracked down his now-fiancee, Amanda, an old friend from Welwyn Garden City, through Friends Reunited is frequently quoted. James divorced from his wife, Tanya, with whom he has four children, in 2005. The settlement was a reputed £3m.

When he promises to introduce me to the Green Goddess, I assume he means Amanda, frequently credited with being behind his green epiphany. But he means a 40-tonne vintage fire engine installed at the top of the vegetable garden. 'It's a piece of history,' he insists. No doubt, I say, but not very green. 'I won't drive it,' he promises. 'Couldn't drive it anyway, because it runs on leaded. It stays there, a piece of history. I'm a bit obsessive about stuff like that.'

Not far away is James's Chrysler, another piece of his automotive history that I wish wasn't there - but he takes a different view: 'I'm a footballer with six kids and a big kit bag; I can hardly drive a Polo.' Besides, as announced in his Observer column a few weeks ago, he has at least tried to green this particular monster engine. He paid £2,500 to Blooming Futures, who specialise in conversions, to adapt it to run on pure (and crucially, local) rapeseed oil.

A slightly bemused-looking TV crew from Sky Sports is also in attendance. 'What have you had done to your car, David?' asks Bianca, the presenter, on camera. James eloquently explains that it ran on rapeseed oil, distinct from biofuels that get a bad press because they are made from palm oil, but concedes, 'It's an imperfect solution because the emissions are not much lower than a diesel, but this is a renewable fuel. I've got the money to change my engine over, so I will.'

Two months on, has the conversion been a success? It's not all good news. 'Er,' he smiles, 'I've lost the key, and anyway it's broken down.' Amanda rolls her eyes. 'It's broken down four times,' she elaborates. Apparently all will be rectified soon. 'It's better I do the trial and error bit,' James says stoically, 'so that the next person gets a problem-free conversion.' He's hoping the next plant oil-burning Chrysler will belong to one of his team-mates, but he admits there's some way to go. When he was talking to Radio 4 about the environment recently, some of his fellow Portsmouth players took delight in revving the engines of their luxury cars in the background.

James remains determined to convert his Pompey team-mates to green issues, but 'not by preaching at them. The older they are, the more receptive they are to green ideas. I don't lecture them, but I have explained what I perceive is happening with climate change and how we could make a difference.' He is fiercely protective of football players, and thinks the criticism they get over wages is unfair. 'Guys in the City can earn more. Footballers' consumerism or emphasis on luxury brands is hardly the root of all environmental problems,' he reasons. 'When you earn money, you want to show it off - everybody does. With my first pay cheque I bought two horrible diamond rings from that really nasty jeweller's that used to be on the high street in Welwyn Garden City.'

Amanda, dressed casually in jodhpurs and T-shirt, seems resolutely non-bling and down to earth. Her mother, a geologist, joins in a conversation about uranium deposits and the feasibility of nuclear power - not standard footballer chat. The only sign of Amanda's Wag status that I can find is a small Baden-Baden fridge magnet, a souvenir from the World Cup.

What about the attitude of David and Victoria Beckham, king and queen of conspicuous consumption? 'More than anyone I know,' says James, 'David is the most philanthropic. He really has done some amazing stuff and if I could replicate something like that in environmental terms, I'd be very pleased.'

His environmental conversion seems to have developed out of his wish to link football with community projects, and a love of finding solutions to problems. 'Amanda's been doing the recycling thing since the off,' he says, 'because in Devon it's been a big thing. Meanwhile I lived on my own on the Wirral [when he was playing for Liverpool and had split up from his wife] and nobody explained about recycling bins. When I came down here to Devon, Amanda was like, "Don't put the plastic in the bin, and this is biodegradable," and I became a bit intrigued. I even tried to invent a special recycling bin - but I got no further than a sketch.'

When he moved to Manchester City, James did some publicity promoting the club's new Ecotricity wind turbine. 'I thought that project was brilliant,' he says. 'It was about community, and to me that's what football clubs are all about. We got clean power for the club, and 4,000 local houses.'

In May 2005 he was invited by the FA to join Rio Ferdinand and Gary Neville on a trip to Malawi to promote HIV and Aids awareness. While he was there he became interested in an entirely different cause. 'There was still famine following a drought in 2005, and crops failing everywhere. And yet Malawi has a lake one-fifth the size of the country. I just couldn't understand why they didn't irrigate it, and why food wasn't being grown differently. Basically all of this was buzzing round my head and I was driving Rio and Gary mad going on about it, but I was trying to tie it into the stuff I'd learnt about renewables from the Man City turbine project, and think how we could get power out there.'

Back on the Wirral, James happened to meet an ex-Malawi agricultural minister (with the unlikely name of Harry Potter) living nearby in Chester, and spent his spare time talking about agrarian production methods. Through Amanda he enlisted the help of local organic farmers in Devon, and plotted with Robert Hillier, the BBC Head of Sport for the World Service, to start the David James Foundation. The foundation's first project has been to work with Australian drought agronomists to showcase different farming methods and educate the Malawian farmers on the frontline of climate change.

'David is never going to be just holding up a T-shirt and smiling for the camera,' says Hillier, who advises on green matters. 'He wants to get deeper into the issues; he's interested in being well-rounded and well-read. When he picks a mission or cause it is never for publicity because, quite frankly, whenever he gets involved in anything he is creating a pain in the arse for himself. After all, nobody wants every aspect of their life under scrutiny all the time. DFID [the Department for International Development] was enormously keen in the beginning, but it was all, "Yes, yes dear boy, come and play a charity football match," and beyond that they wouldn't really engage.' James agrees: 'In the end it was easier for me and Robert to set up my foundation.'

James gets consumed by his projects. 'When I was recalled for England it clashed with a trip to Malawi, so I suggested to Robert that I wouldn't play for England. He put me right on that straightaway,' he laughs. 'You're always much more use as an England player than just a Premiership one. And call it serendipitous, for me rather than him, but [goalkeeper] Chris Kirkland actually broke his finger just before the World Cup, so I got back in.'

So when it comes to the green agenda, James needs to make hay while the sun shines, or rather while he's still high profile - after all, he is 36. Signed to play for another year with Portsmouth, he doesn't foresee a post-football career in TV punditry. 'To be honest,' he says, 'I don't know the names of half the players I play against these days.' Not that his need to achieve has diminished - he is determined to play for England again - a need he charts back to a poor childhood, briefly in Jamaica and then Welwyn Garden City, where he felt adrift from his contemporaries. His parents separated when he was two and he and his younger brother, Gilbert, moved to his maternal grandparents' house in Hatfield. It was through athletics and football, at Sir Frederic Osborn School, that he marked himself out as an achiever.

Watching An Inconvenient Truth hammered home the urgency of promoting environmental awareness. When Portsmouth announced their new ground designed by Swiss architects Herzog & De Meuron he saw it as a chance to make it a green ground. 'Initially I said, "What about the environment?" and the club replied, "Well you won't be able to see it from the road," which wasn't really what I was getting at. But now we're having lots of conversations.'

He sees scope for addressing waste, water and energy use and harnessing renewables in the new build. James is also keen to look at the way fans get to and from matches, including subsidised rail travel as a low-carbon alternative to car travel. On all of this he seems to be rather ahead of the official curve. Dan Johnson from the Premier League promises the football industry is catching up. 'We were looking at this subject anyway,' he says, when asked about greening up the beautiful game, 'and clearly David would be a great person to get involved - because he's obviously very committed - when we've got a clear picture of where we are.'

Where they are is right at the beginning of the process, having commissioned the environmental consultancy Beyond Green to audit all 20 Premiership clubs. 'We are auditing the Premier League as a precursor to developing a sustainable strategy,' confirms Jonathan Smales, the firm's director. 'So we're looking for and finding good practice, which is very heartening. I think this is a natural progression for football, following campaigns such as Kick Out Racism and supporting the nurses' campaign for better wages. Caring for the environment shows a new, attractive face to football and the Premier League. David James has been a pioneer in all of this. He's informed, outspoken and articulate and he's giving football a good name. But when we get them out of their 4x4s and sports cars and on to bikes we'll know there's been a real revolution on and off the field.'

James is not yet on his bicycle but is well-placed to take on the green cause, not least because he never assumes knowledge but treats it all as a learning experience. 'I wish someone could sit me down and tell me conclusively exactly who is responsible for climate change, the best way to tackle emissions and how fast we need to cut rates. I want those conclusive instructions,' he says. 'But it's not like that. You have to assess the information you get and do what you can. My attitude is, if we are causing even just a tiny fraction of the problem, I'm going to do something positive to counteract that.'

'Of course there will be people thinking he's off on one again,' says Hillier of James's new-found mission. 'They're usually the people who like footballers to fit a mould and thought Graeme Le Saux was gay because he read the Guardian. Besides, you build up a certain resilience after years of being called names.'

James was once dubbed a 'Spice Boy' along with Robbie Fowler and Steve McManaman at Liverpool - the suggestion being that they partied so hard that they lost games. A subsequent bad patch on the field meant that the Spice Boy tag was replaced with 'Calamity James'. I suggest his new incarnation will have an eco theme - tabloids through to Radio 4 have picked up on his green leanings. Perhaps, I suggest, he should be known as 'Climatey James'? For the first time all day, he looks unenthused.